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Development of Clear Recovery Area Guidelines (2024)

Chapter: Chapter 1 - Introduction

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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2024. Development of Clear Recovery Area Guidelines. Washington, DC: The National Academies Press. doi: 10.17226/27593.
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3   Introduction The clear zone concept for roadside design emerged in the mid-1960s. It was defined as a recovery area that afforded a driver a reasonable opportunity to regain control of an errant vehicle and avoid a crash. It originated as a single lateral distance beyond which a vehicle striking a roadside obstacle was less probable. The default clear zone distance was 30 ft. If obstacles within this lateral distance could not be removed, some form of shielding or protection was introduced. Acceptance of a single distance for lateral clearance diminished over time. On low-volume, low-speed facilities, the 30-ft lateral clear recovery distance was considered excessive. On the other hand, it was recognized that some roadside slopes could result in increased encroachment distances, and a 30-ft clear zone might be insufficient. Subsequent recovery area guidance contained in the American Association of State Highway and Transportation Officials (AASHTO) 1977 Guide for Selecting, Locating, and Designing Traffic Barriers (Barrier Guide) (1) and the AASHTO 1989 Roadside Design Guide (RDG) (2) was expressed in terms of traffic volume, design speed, sideslope, and other roadway and roadside factors. The clear zone guidance has remained relatively unchanged since those times. The minimum recom- mended clear zone distance range is 7 to 10 ft, corresponding to a design speed ≤ 40 mph, design average daily traffic (ADT) under 750, and a 1V:4H or flatter foreslope. Conversely, the largest recommended clear zone range of 38 to 46 ft corresponds to a design speed of 65 to 70 mph; a design ADT over 6,000; and a foreslope ratio of 1V:5H to 1V:4H. Although these guidelines provide a more realistic approach than the application of a single distance, the values are based on studies from the 1950s through the 1980s that used relatively limited data and extrapolated numbers. Further, transportation agencies frequently face difficul- ties in providing desirable recovery areas because of right-of-way constraints or construction costs. Updated guidelines are needed to aid designers in understanding the risk while recogniz- ing and working within the associated constraints. The objective of this study was to develop updated guidelines for roadside clear zones expressed in terms of key roadway and roadside design parameters. This report documents the research per- formed and clear recovery area guidelines developed under this project. The research approach combined crash data with vehicle dynamics computer simulation results to consider a wide range of encroachment and design variables. This data was used to develop clear zone guidelines through an encroachment probability-based, relative risk analysis methodology. Chapter 2 provides a historical overview of clear recovery area guidance in the United States. Details of the vehicle dynamics encroachment simulations are presented in Chapter 3. The vehicle dynamics simulation matrix consisted of over two million unique vehicle encroachment simula- tions. The simulations were executed using a state-of-the-art, multi-rigid-body vehicle dynamics code, modified by the project researchers, to include a friction ellipse model to simulate soil C H A P T E R   1

4 Development of Clear Recovery Area Guidelines furrowing during vehicle sideslip and a vehicle-body-to-terrain contact algorithm to model con- tact between vehicle hardpoints and the roadside terrain. Simulation output included lateral distance traveled, vehicle stability outcome, trajectory data, and velocity data. Chapter 4 documents the development of encroachment variable distributions and marginal probabilities for the values of the encroachment variables used in the simulation matrix. NCHRP Project 17-43, “Long-Term Roadside Crash Data Collection Program” crash database, published as NCHRP Web-Only Document 341: Roadside Database Coding Manual, was used to develop univariate distributions for key encroachment parameters (3). The research team used these distributions to determine conditional probabilities or weighting factors for the values of the variables used in the simulation matrix. A probability matrix for vehicle type was developed using vehicle sales data by combining sales percentages for the vehicle makes and models cor- responding to the platforms of the simulated vehicles. Chapter 5 describes the development of encroachment relationships from the weighted simula- tion results. Statistical models were derived in terms of significant roadway and roadside design variables to assist with determining the probability of an impact given an encroachment has occurred and the severity of the impact. The development of a risk analysis tool, referred to herein as the Clear Zone Guideline Assis- tance Program (CZ-GAP), is presented in Chapter 6. The encroachment relationships developed from the simulation results were incorporated into the risk analysis tool to estimate the prob- ability of a fatal or serious injury crash [P(K+A)]. An encroachment-based analysis method estimates the conditional probability of a crash given a roadside encroachment has occurred and the probable severity of the crash. Chapter 7 documents analyses performed to evaluate the sensitivity of encroachment and design variables to the estimation of risk and the relative importance of variables to the overall determination of P(K+A), which is the basis for the development of clear recovery area guide- lines. Several design configurations were selected, and the values of individual design param- eters were varied to examine the effect of changes in these parameters on risk. The results of the sensitivity analyses were used to retain or exclude variables from the clear zone guideline development process. The recovery area guideline development process is described in Chapter 8. The CZ-GAP risk analysis tool was used to estimate P(K+A) for various roadway and roadside design configu- rations. Analyses were parametrically executed to cover combinations of facility type, posted- speed-limit categories, roadway and roadside design variables, clear zone distance, and hazard conditions beyond the clear zone edge. A relative risk approach was used to define a clear zone distance for a given design configuration that has a P(K+A) risk equal to that of the guardrail. The final clear recovery area guidelines will be made available for consideration for incorpo- ration into the AASHTO RDG. The guidelines can be used to determine a recommended clear recovery distance for a given set of roadway and roadside characteristics. They also consider the nature of the hazard that exists beyond the clear zone edge. Finally, Chapter 9 presents study conclusions, recommendations, and suggested future research.

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The clear zone concept for roadside design emerged in the mid-1960s as a single distance for lateral clearance that reduced the likelihood of an errant vehicle striking a roadside obstacle. Subsequent recovery area guidance that evolved over the next two decades provided a variable distance expressed in terms of traffic volume, design speed, sideslope, and other roadway and roadside factors.

NCHRP Research Report 1097: Development of Clear Recovery Area Guidelines, from TRB's National Cooperative Highway Research Program, develops updated guidelines for roadside clear zones expressed in terms of key roadway and roadside design parameters. These updated guidelines can aid designers in better understanding the risk associated with roadside encroachments while recognizing and working within the associated design constraints.

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